1. Field of the Invention
This invention relates to a method of recording or instantaneously visualizing enlarged images which are "in relief" or stereoscopic images of objects, and to an optical apparatus implementing this method.
The expression "enlarged images of objects" is understood as designating images, the dimensions of which are of the same order of magnitude as or are greater than those of the object.
The expression "stereoscopic image" or image "in relief" is understood as designating an image which allows an observer thereof to have a different vision of the same object which is represented corresponding to different observation angles thereof, i.e. corresponding to a changing perspective of the object as a function of the position of the observer with respect to the image. In other words, during visualization, images of this type make it possible, on the one hand, to observe the object under changing aspects as a function of the observation angle and, on the other hand, even if the observer is immobile, to create an impression of relief due to binocular vision because, as the two eyes of the observer receive slightly different images of the same object, the brain is capable of reconstructing the spatial volume.
2. Description of the Prior Art
Different devices are already known which allow the production of such images which give an impression of relief during the restitution or the observation of the image. To this end, a lenticular grating is usually used which is formed from a series of thick elementary lenses which are spherical or preferably cylindrical and are juxtaposed. Thus, cylindrical lenticular gratings are usually in the form of a plate of transparent material, one face of which is grooved and the other is flat.
Lenticular gratings of this type which are formed from a series of juxtaposed, cylindrical, elementary lenses dissect or sample the images which impinge thereon into a series of extended elementary beams. Thus, the final resolution depends on the width of each lens.
An incident beam of parallel light impinging on the convex face of the lenticular grating is focussed at the level of the focal plane of the lenticular grating common to all the cylindrical lenses, a focal plane which generally corresponds to the flat face of the grating in the form of a family of very fine lines at a rate of one line behind each of said lenses. A second beam of parallel light, having an angle of incidence which is slightly different from the first beam, and consequently corresponding to a different observation angle of the object, is dissected and focussed into a second family of lines which are juxtaposed to the previous lines. Each family of lines which is formed in the focal plane of the lenticular grating corresponds to a particular direction of the incident light. The image thus formed in the focal plane of the lenticular grating is termed a "composite image".
Thus, the composite image is formed from a series of families of lines, the position of which depends on the direction of the incident beam.
Of course, the width of each line depends on the defects of the cylindrical lenses, such as aberrations, astigmatism, curvature of field, and diffraction.
In the prior art optical apparatus for the formation of images in relief, it has proved to be necessary to use a plurality of lenses or other optical devices (such as so-called Wollaston or Dove prisms) which use particularly complicates the optical devices and renders their production delicate, or it has proved necessary to perform a movement, either of the object, or of the apparatus, or of both simultaneously, in order to at least approximately cover each of the zones located at the level of the focal plane behind the different lenses of the lenticular grating. To this end, it is necessary for the relative movement to return to sweep over in the image space an angle substantially corresponding to the angle of field of the lenticular grating, i.e., the angle which determines an angular zone, inside which each direction of the incident beam of parallel light provides a specific family of image-lines at the level of the focal plane of the system. Any ray which departs from the angle of field of the lenticular grating will no longer be focussed on the part of the focal plane located just behind the penetrated cylindrical lens, but will be focussed on another part corresponding to an adjacent cylindrical lens. Thus, this angle of field should not be exceeded in order to avoid superpositions and interference of the information recorded in the form of lines.
Said movement between the object and the lenticular grating may comprise, for example a rotation of the exposure apparatus around the object, a translation of the lenticular grating, or a simultaneous rotation of the object and the lenticular grating.
French Pat. No. 943601 describes a device for the production of photographs in relief which may be used in particular for macrophotography and which uses this last type of movement. The device described in this patent comprises two platforms which are capable of rotation about an axis, and respectively support the object and the assembly of lenticular grating and photographic emulsion. The lens is stationary, and bellows ensure optical tightness between it and the lenticular grating. The two platforms are synchronous and rotate during the complete duration of the exposure determined by the shutter aperture, at an angle equal to the angle of field of the lenticular grating. Of course, the speed of rotation depends on the lighting conditions. The brighter the lighting, the faster the speed. Moreover, it is possible to change the magnification by varying the distance between the object and the lens, and the extension of the apparatus.
However, if apparatus of this type function correctly, they necessitate, due to the movement which is to sweep over the angle of field of the lens-shaped system, relatively long exposure times, thereby preventing the exposure of moving objects and dynamic phenomena.
Moreover, conventional apparatus usually provide, in a first stage, an inverted relief image. In order to correct this phenomenon of pseudo-stereoscopy, provision is made during visualization to either re-invert the relief of the composite image by means of a supplementary stage which comprises re-photographing the assembly of negative lenticular system with relief inversion, or inserting into the visualization device optical systems, such as lenticular gratings, which are to re-invert the relief during observation. These different solutions greatly complicate the optical device and are not completely satisfactory.
In particular, in conventional apparatus, the considerable spread of the beam necessitates the use of at least three lenticular gratings to obtain orthostereoscopy.